Image reading devices of the type that allow optimal imaging distances, include a targeting scheme of some kind useful solely in aligning the image reader to the target to obtain an optimal imaging distance from the target. Most of these techniques project a pattern onto the target from a targeting LED or laser diode. An aperture and light shaping optics are often used to project a pattern onto the target as an aid to the human operator to correctly position the image reader based on the targeting pattern.
The operator moves the image reader until targeting lines or dots converge at a predefined point, which relates to the point at which an acceptable image can be acquired. An acceptable image may be defined as one that contains sufficient resolution for post-processing. With this method, it may be difficult for the operator to determine which direction the device must be moved. Further, with this method, the operator can only capture an image at the predetermined distance at which the lines or dots converge, even though an acceptable image could potentially be acquired some distance before or beyond that point.
Many image readers use a single laser as a targeting pattern source, but single laser systems often suffer from parallax error. This refers to the discrepancy between the framing of an image and what is actually imaged. This is due to the fact that the laser and the line of sight of the lens are not aligned.
U.S. Pat No. 5,500,702, which issued to Meyers on Mar. 19, 1996 and European Patent EP1172756 to Datalogic S.P.A. both discuss the use of a refractive or diffractive element placed along the path of the single laser beam to create a multi-beam pattern. This pattern is intended to frame a target. Due to parallax error, however, any time the target is scanned from an angle, the targeting pattern skews and stretches and the error increases as the symbology reader diverges from the perpendicular. Also, the scan is only accurate at a particular distance.
Ideally, a targeting pattern should be projected from an optical source that is concentric with the optical center of the imaging lens. This assures accurate alignment of the pattern with the symbology being targeted. This approach, however, is impractical and complex.
Therefore there is a need for a targeting system that has a targeting pattern which is easy to use, is unaffected by errors of parallax, and yet is simple to implement.